Whaat is the difference between the above and would we use each one to enhance our Dancing

Sorry for the typo. How would we use each one to enhance our Dancing. Cheers

Hi Doug
THis is not easy to answer, but i'm used to putting my neck into a noose, so here goes
Rhythm is essentially what you would tap out with your foot while the music is playing. Of course there could be syncopation. If you listen to the start of ,say, "sing-sing-sing" (a great quickstep) the drummer is putting in extra hits on half notes and the like, but there is a clear underlying rhythm which you can tap out.
Melody is what you would whistle. For example the start of "night and day" would be easy enough to whistle and most would recognise it.
Harmony - this is much more difficult to explain. I used "night and day" especially because the very start has a fancy chord accompaniment which gives the music a sad, sombre feeling. Music has a chord sequence and a melody line, and the interplay of these is, in general terms, harmony. It's a complex thing, and hard to explain without playing music.
How to use them??
Better dancers than me could give better answers. Rhythm determines the general tempo of your dancing, but a really good dancer could, for examle, put in extra scatter-chasses in quickstep if there were syncopation which would justify it.
Harmony would modify the 'feel' of a piece, and a good dancer would put this feeling into his dancing.
A 'happy' foxtrot, such as "you make me feel so young" would give a dancer a completely different feeling from 'night and day' which is a much more sombre piece. However, it takes a very good dancer to externalise that feeling into something you can see in their dancing.
Anyone have any ideas before the noose tightens around my neck too much!

All my references to "night and day" should have been to "dancing in the dark". Apologies

Actually, almost everything you say could perfectly well apply to Night & Day.

Think of the introduction - about as tuneful as the One Note Samba - the harmony is everything. And it is hardly a "happy" number. The original Fred Astaire version misses almost everything that makes this a truly great song, but if you are curious, seek out the version sung by the English singer/pianist Peter Skellern, who did a whole album called "Astaire", which, while not in strict tempo, and therefore crap for dancing (unless you could adapt the Argentine Tango rhythmic freedom to ballroom standards - which would be very interesting both to dance and to watch!), are probably the best interpretations of these classic songs that I have ever heard. The punchline is that his orchestra is actually a British Brass Band, which is about the most bizarre combination possible - but the result nearly takes your breath away, and is just on a different plane from most versions you will ever hear.

The qustion was Rhythm. Harmomy Melody.and would we use each of these in dancing. Just my thoughts. Harmomy can be forgotten it doesn't count. Melody. Isn't that the song that is being played. Rhythm Isn't that the spacing of the music, the difference between a Waltz and a Tango. Harmony is out. Melody. I couldn't care less what they play. Usually there is no choice anyway, so that's out. All I am left with is Rhythm of which I can be out of Rhythm or in Rhythm. What should be added to that list is timing. It is possible to be in time but out of rhythm. Which leads to being out of the comp. That is dancing 3 4 1 2. and not correctly 1 2 3 4.

You could use the melody to express the music. If the melody soared [without or without a tempo change} you might dance a particular figure to enhance or match the soaring of the melody. And key changes [which are related harmony}are musical transitions that the musical dancer would use to his advantage.
Bing Crosby sings a duet with someone {I forget who} that is a great quickstep and the melodies intertwine and create harmonies that beg for dance expression. How the dancer hears and expresses the music is what makes everyone different.